In vitro assays used to evaluate the ability of a microbicide to prevent HIV transmission typically utilize models more relevant to the development of anti-HIV therapeutic agents and quantify virus production at short time intervals following infection (3-6 days) using insensitive methodology. To mitigate the shortcomings of these critical biological assays, we have developed a microbicide transmission and sterilization assay (MTSA) to more sensitively and quantitatively evaluate virus transmission in cell culture in the presence of microbicidal compounds. The MTSA defines the concentration of the microbicide required to totally suppress the transmission, replication, and spread of virus in cell culture and may thus help define the effective concentration of the microbicide required to be present in target cells upon delivery in a formulated microbicide product. In light of the difficulty of quantifying the dose of a microbicide product that is required for the delivery of an effective product, it is important to understand how in vitro methodology, complemented with appropriate ex vivo and in vivo pharmacokinetic and pharmacodynamics evaluations, might inform the development process and serve to better define appropriate dosing strategies. Among microbicide developers, the dosing problem is now addressed through the philosophy of more is better or dose with as much API as can be effectively and safely delivered. This dosing philosophy yields significant problems, including potential toxicity and irritation, API solubility, and the overall ability to effectively develop and manage the formulation or delivery vehicle for the microbicide. Thus, a better understanding of means to define the dosing needed in the context of product efficacy and toxicity, permeability of the product to tissue, the biophysical and biochemical attributes of the formulated product, and the ability of the API to achieve sterilizing concentrations in target tissues (PK/PD) must be addressed. Preliminary data with a variety of microbicide products suggests the MTSA may resolve critical dosing determination issues by defining the dose necessary to total suppress virus transmission and replication, and will provide a highly quantitative biology-based methodology to prioritize and appropriately deliver microbicides.